No, the prosecution bears the burden of proof that you were found in possession of a semi-auto rifle configured as an "Assault Weapon."

It is then YOUR burden to prove that your rifle qualified to be legally grandfathered under the law.

Mac,

As long as BATF can't *prove* that the gun belonged to someone else on 9/13/94, then this letter should be good enough.

-Troy

That's true, but it would be far more difficult to convince a jury that a person knowingly violated the law if he had a letter from the person who owned the gun on 9/13/94.  Your point about not having to have owned the gun, but still being familiar with its configuration is valid.  Basically, the letter would be stating:

"I, so and so, have first-hand knowledge that gun xxx, serial number ########, was in 'Assault Weapon' configuration on 9/13/94, and was legally possessed.  I bear full witness to these facts."

Or something like that.  The point is, someone who didn't own the rifle on 9/13/94, and had no first-hand knowledge of its configuration on or before the date, could not truthfully provide such a letter.  A lot of undocumented pre-bans have changed hands since 9/13/94...

-Troy